The present invention relates to the field of seismic exploration. More particularly, the invention relates to a method and apparatus for seismic exploration, and most particularly to deployment and retrieval of ocean bottom seismometer systems in deep marine environments.
Seismic exploration generally utilizes a seismic energy source to generate an acoustic signal that propagates into the earth and is partially reflected by subsurface seismic reflectors (i.e., interfaces between subsurface lithologic or fluid layers characterized by different elastic properties). The reflected signals (known as “seismic reflections”) are detected and recorded by seismic receiver units positioned at or near the surface of the earth, thereby generating a seismic survey of the subsurface. The recorded signals, or seismic energy data, can then be processed to yield information relating to the lithologic subsurface formations, identifying such features, as, for example, lithologic subsurface formation boundaries.
Typically, the seismic receiver units are arranged in an array, wherein the array of seismic receivers consists of a single string of receivers distributed along a line in order to record data from the seismic cross-section below the line of receivers. For data over a larger area and for three-dimensional representations of a formation, multiple strings of receivers may be distributed side-by-side, such that a grid of receivers is formed.
While the fundamental process for detection and recording of seismic reflections is the same on land and in marine environments, marine environments present unique problems due to the body of water overlaying the earth's surface. In marine environments, even simple deployment and retrieval of seismic receiver units is complicated since operations must be conducted off the deck of a seismic exploration vessel, where external elements such as wave action, weather and limited space can greatly effect the operation. These factors have become even more significant as exploration operations have pushed to deeper and deeper water in recent years.
Exploration in deep water has resulted in an increased reliance on seismic receiver units that are placed on or near the seabed. These devices are typically referred to as “OBC” (Ocean Bottom Cabling) or “OBS” (Ocean Bottom Seismometer) systems. There are three main groups of ocean bottom apparatus generally used to measure seismic signals at the seafloor. The first type of apparatus is an OBC system, similar to the traditional towed streamer, which consists of a wire or fiber cable that contains geophones and/or hydrophones and which is laid on the seabed, where the detector units are interconnected with cable telemetry. For OBC systems, a seismic vessel will deploy the cable off the bow or stern of the vessel and retrieve the cable at the opposite end of the vessel. In most cases, three ships are required to conduct the overall operation since, in addition to a seismic energy source vessel, a specially equipped vessel is necessary for cable deployment and a separate vessel is needed for recording. The recording vessel is usually stationary and attached to the cable, while the deployment vessel is generally in constant motion along the receiver line deploying and retrieving cable. Because the recording vessel is in constant physical contact with the cable, the effort required to maintain the vessel's position to counter wave action and ocean currents can generate great tension within the cable, increasing the likelihood of a broken cable or failed equipment, as well as the introduction of signal interference into the cable.
A second type of recording system is an OBS system in which a sensor package and electronics package is anchored to the sea floor. The device typically digitizes the signals and uses a wire cable to transmit data to a radio unit attached to the anchored cable and floating on the water surface. The floating transmitter unit then transmits the data to a surface vessel where the seismic data are recorded. Hundreds if not thousands of OBS units are typically deployed in a seismic survey. A third type of seismic recording device is an OBS system known as Seafloor Seismic Recorders (SSR's). These devices contain the sensors and electronics in sealed packages, and record seismic data on-board while deployed on the seafloor (as opposed to digitizing and transmitting the data to an external recorder). Data are retrieved by retrieving the device from the seafloor. SSRs are typically re-usable.
Each OBS system generally includes components such as one or more geophone and/or hydrophone sensors, a power source, a crystal oscillator clock, a control circuit, and, in instances when gimbaled geophones are used and shear data are recorded, a compass or gimbal. Many of these components are subject to various effects arising from the orientation of the OBS unit as it is deployed on the ocean bottom. For example, crystals are subject to gravitational effects, such that orientation of the OBS system can effect operation of a crystal clock. Any misorientation of the OBS system on the seabed can result in clock inaccuracies. Likewise, while mechanical gimbals may be used to correct for tilt, pitch in many mechanical gimbal devices is limited 30°. For such devices, in order for the gimbal to function properly, the OBS system must be deployed on the seabed in substantially the desired orientation, i.e., approximately 30° from the horizontal or less. Of course, it is well known that mechanical gimballing of a geophone is expensive and requires more space than a non-gimballed geophone, and as such, it is desirable to deploy an OBS system so as to render gimballing unnecessary.
As with orientation, the location of OBS system on the seabed is necessary to properly interpret seismic data recorded by the system. The accuracy of the processed data depends in part on the accuracy of the location information used to process the data. Since conventional location devices such as GPS will not operate in the water environments, traditional prior art methods for establishing the location of the OBS systems on the seabed include sonar. For example, with a sonar system, the OBS device may be “pinged” to determine its location. In any event, the accuracy of the processed data is directly dependent on the precision with which the location of the OBS system is determined. Thus, it is highly desirable to utilize deployment methods and devices that will produce dependable location information. In this same vein, it is highly desirable to ensure that the planned positioning of the OBS device on the seabed is achieved.
One problem that is common in all types of seismic systems physically deployed on the seabed is the degree of coupling between the system and the seabed. Those skilled in the art will understand that the physical coupling between a seismic unit and the earth has become one of the primary concerns in the seismic data collection industry. Effective coupling between the geophones of a system and the seabed is paramount to ensuring proper operation of the system. For example, in an OBC system where three-dimensional geophones are employed, because the cable is simply laid on the seabed, geophones are not rigidly coupled to the sediment on the seabed. As such, horizontal motion other than that due to the sediment, such as for example, ocean bottom currents, can cause erroneous signals. Likewise, because of its elongated structure, OBC systems tend to have satisfactory coupling only along the major axis of the cable when attempting to record shear wave data.
To enhance coupling, many OBS systems of the prior art separate sensing units, i.e., the geophone package, from the remainder of the electronics to ensure that the geophones are effectively coupled to the seabed.
Thus, orientation, positioning and coupling of an OBS unit are all important factors in achieving effective operation of a seismic unit and collection of seismic data. Each of these placement components is highly dependent on the manner in which the OBS units are deployed. Typically, for operations in coastal transition zones such as shallow water or marshes, units are simply dropped in a water column over the side of a deployment vessel above the targeted seabed position. Because the water column is comparatively shallow and the OBS unit is relatively heavy, the effects of ocean currents, drag and the like is minimal and the desired positioning of the OBS unit on the seafloor can be fairly easily achieved. In contrast, an OBS unit dropped through hundreds or thousands of feet of water and subject to the forces of buoyancy, drag and ocean currents could settle on the seabed as much as several hundred feet from its original position. Not only is the unit likely to be of little value in the seismic survey because of its misplacement, locating and retrieving the OBS unit becomes much more difficult.
Of course, orientation is often less certain than positioning. Various objects, whether rocks, reefs or even discarded debris, can disrupt the desired orientation of a unit, which in most cases is preferably parallel with the horizontal. Those skilled in the art will understand that orientation can effect data accuracy. The most accurate data is data that has been processed to account for the orientation of the seismic collection unit that acquires the raw data. Such processing typically necessitates additional equipment on-board the unit itself to determine x, y and z orientation, as well as additional computational power and time during processing of the raw data.
Likewise, the degree of coupling between a seismic collection unit and the sea floor, whether in shallow water or deep water, is often difficult to determine at the time a unit is positioned. This is particularly true of seismic units that are simply allowed to settle where they land. In many cases, the top layer of silt at a particular location on the sea floor may be somewhat unstable or mushy, such that seismic energy transmission therethrough to the seismic unit is attenuated or distorted in some way. Even in the case of relatively hard or compact sea floors, if a seismic unit has not formed a good coupling with the earth, seismic energy passing from the earth to the unit's geophones may be attenuated. Thus, even if a unit is positioned in the desired location and oriented to minimize gravitational effects and the like on seismic data, a high degree of coupling must still be achieved in order to maximize the quality of the collected data.
Thus, based on the foregoing, it is highly desirable to be able to place OBS units on the sea floor of deep water locations so as to ensure the desired positioning and orientation is achieved and to maximize coupling between the unit and the sea floor.
Because the push to conduct seismic operations in deep water is relatively recent, few attempts have been made to address the above-mentioned problems associated with deep water deployment of OBS units. U.S. Patent Application Publication U.S. 2003/0218937 A1 discloses a method for OBS deployment utilizing a tethered remote operating vehicle (“ROV”) and a separate OBS carrier cage, each lowered to the seabed on a separate line. The carrier contains a plurality of OBS units. The reference teaches that once the ROV and carrier cage are positioned adjacent the seabed, the ROV can be used to extract and place each OBS unit in the desired location around the carrier. In a preferred embodiment, a plurality of carriers are used to simultaneously lower a large group of OBS units close to the seabed at one time so as to maximize the operational time of the ROV.
Those skilled in the art will understand that such a system will likely encounter operational problems in light of the rigors of deep water operations where extreme depths, surface conditions, multiple ocean currents and mushy or unstable sea bottom conditions can all significantly affect the deployment effort Most notably, the drag on the carrier, the ROV and their respective lines are all different, and as such, these different components of the deployment system will have disparate responses under water when subject to the various elements. In the case of the carrier cage, there is no mechanism for remotely controlling the position of the cage in the water, the result being that the cage is highly likely to be pulled along in the direction of the prevailing ocean currents with very little control over the cage's movement. In this same vein, the tether for the ROV and the line for the carrier cage are likely to be of different dimensions and buoyancies such that drag on the lines is likely to differ substantially. When deployed in thousands of feet of water, the effects of drag on these various elements of the prior art system are significantly magnified, such that the ROV and the carrier cage could be hundreds of feet or more apart.
Perhaps even more threatening to such a system under actual operating conditions is the likelihood that the lines will become tangled, interrupting a seismic shoot and threatening profitability. Those skilled in the art understand that as the number of lines in the water at any given time increases, the more complicated the operation becomes and the more difficult it becomes to control movement of the lines and prevent tangling. This is particularly true where the lines, as well as the objects attached at the lower ends of the lines, have different drag characteristics. Even when the end of one line is controlled by an ROV, but the other is not, entanglement is likely. As an example, each line may be as long as 10,000 feet extending from the surface of the water to the seabed. Since a typical deployment boat may be only 40 feet wide and each line is deployed on opposite sides of the boat, there is a high probability that the carrier cage line will become entangled with the ROV tether.
An additional drawback to the above-described prior art system is that it utilizes only a single ROV for deployment and retrieval. While such a system minimizes the cost of operations, the entire operation is dependent on the operability of the single ROV. Any breakdown of the ROV can substantially delay the deployment/retrieval efforts since repairs would be necessary before continuing.
Thus, it would be desirable to provide a deployment system for deep water seismic data collection units that minimizes the effects of drag, weather, ocean currents, depth and similar conditions on OBS deployment operations at or near the seabed. Additionally, such a system preferably would be disposed to retrieve previously deployed OBS units. Such a system should permit accurate placement and orientation on the seabed and good coupling of individual OBS units therewith. Preferably, such a system would utilize an ROV to deploy and/or retrieve multiple OBS units at or near the seabed in a manner that maximizes use of the ROV in the operations. The system should provide minimum likelihood of entanglement of lines if more than one line is used in the same operation. The system should also provide for efficient turn-around of OBS units that have been retrieved from deployment. Such turnaround would desirably include data extraction and recharging of the OBS units. In the preferred embodiment, the deployment system would also minimize the effects of equipment breakdown on the overall seismic acquisition operation.
OBS units deployment should be easily accomplished, yet the OBS units should be deployable at a certain location with a high degree of confidence.
The system should also be capable of readily handling the hundreds or thousands of OBS units that comprise an array for deployment in ocean environments. Such a system should be able to deploy, retrieve, track, maintain and store individual recorder units while minimizing manpower and the need for additional surface vessels. The system should likewise minimize potential damage to the individual units during such activity.